In the class of hearing aids generally referred to as implantable hearing aids, some or all of various hearing augmentation componentry is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process. Implantable hearing aids may be generally divided into two sub-classes, namely, semi-implantable and fully implantable. In a semi-implantable hearing aid, components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully implantable hearing aid, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system to cause or enhance the sensation of sound for a patient.
A number of different types of implantable transducers have been proposed. By way of primary example, such devices include those that utilize a driver, e.g., an electromagnetic or piezoelectric driver, to move an actuator designed to stimulate the ossicular chain of a patient. By way of example, one type of electromechanical transducer includes a driver that moves an actuator positioned to mechanically stimulate the ossicular chain of a patient via axial vibratory movements. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate, thereby stimulating the cochlea through its natural input, the oval window. As may be appreciated, the utilization of implantable transducers of the above-noted nature entails surgical positioning of the actuator within the mastoid process of a patient's skull. Such positioning typically requires the insertion of the transducer through a hole drilled in the mastoid process. Then, a distal end of the actuator is located adjacent a desired location along the ossicular chain (e.g., interfaced with the incus) or outside the cochlea to mechanically stimulate the same.
Precise control of the interface between the actuator and the ossicular chain is important, as the axial vibrations are only efficiently communicated when an appropriate interface exists, e.g., preferably a low mechanical bias or “optimal energy transfer” interface,” between the actuator and the ossicular chain. Overloading or biasing of the interface can result in damage or degraded performance of the biological aspect (movement of the ossicular chain) as well as degraded performance of the mechanical aspect (movement of the actuator). Similarly, underloading or insufficient engagement between the actuator and the ossicular chain can result in a degraded performance or loss of performance.
In this regard, patients may also experience a “drop-off” in hearing function after implantation due to changes in the physical engagement or interface between the actuator and the ossicular chain due to aspects such as tissue growth. After implantation, however, it is difficult to readily assess the interface between the actuator and ossicular chain without invasive and potentially unnecessary surgery.